It is known that the ability of the human eye to accommodate, i.e., to alter the focal length of the natural lens in the eye, is gradually diminished with increased age. Accommodation in human beings is reduced to 3 D (diopters) or less at an age range of 35-45 years. At that point, reading glasses or some other form of near vision correction becomes necessary for the human eye to be able to bring near objects (such as lines of text in a book or a magazine) to focus. With further aging, accommodation drops below 2 D, and at that point visual correction when working on a computer or when performing some visual task at intermediate distances is needed.
For best results and for best visual comfort, it is necessary to bring each eye to focus on the same viewing target, e.g., a computer screen. A large segment of population requires a different visual correction for each eye. These people, known as anisometropes, require different visual correction for each eye in order to achieve maximum visual comfort while reading or working on a computer. It is known that, if each of the two eyes of anisometropes is not brought to focus at the same viewing plane, the resulting anisometropic image blur causes a loss of stereopsis (depth perception). Loss of stereopsis is one of the best indications of loss of binocular function. Loss of binocularity at the reading plane may cause a drop in reading speed and rate of comprehension, and may hasten the onset of fatigue upon sustained reading or working on a computer. Reading glasses fitted with individually adjustable liquid lenses are therefore uniquely suited for the visual need of individuals with loss of binocular function.
Variable focus lenses can take the form of a volume of liquid enclosed between flexible, transparent sheets. Typically, two such sheets, one forming the lens front surface and one forming the lens back surface, are attached to one another at their edges, either directly or to a carrier between the sheets, to form a sealed chamber containing the fluid. Both sheets can be flexible, or one can be flexible and one rigid. Fluid can be introduced into or removed from the chamber to vary its volume, and, as the volume of liquid changes, so does the curvature of the sheet(s), and thus the power of the lens. Liquid lenses are, therefore, especially well suited for use in reading glasses, that is, eye glasses used by presbyopes for reading.
Variable focus liquid lenses have been known at least since 1958 (see, e.g., U.S. Pat. No. 2,836,101, to de Swart). More recent examples may be found in Tang et al, “Dynamically Reconfigurable Liquid Core Liquid Cladding Lens in a Microfluidic Channel”, LAB ON A CHIP, Vol. 8; No. 3, pp. 395-401 (2008), and in International Patent Application Publication No. WO 2008/063442, entitled “Liquid Lenses with Polycyclic Alkanes”. These liquid lenses are typically directed towards photonics, digital phone and camera technology, and microelectronics.
Liquid lenses have also been proposed for consumer ophthalmic applications. See for example, U.S. Pat. No. 5,684,637 and No. 6,715,876 to Floyd, and U.S. Pat. No. 7,085,065, to Silver. These references teach pumping of liquid in or out the lens chamber to change the curvature of an elastic membrane surface, thus tuning the focus of the liquid lens. For example, U.S. Pat. No. 7,085,065, entitled “Variable Focus Optical Apparatus”, teaches a variable focus lens formed from a fluid envelope comprising two sheets, at least one of which is flexible. The flexible sheet is retained in place between two rings, which are directly secured together, such as by adhesive, ultrasonic welding or any similar process, and the other, rigid sheet may be directly secured to one of the rings. A hole is drilled through the assembled lens to allow the cavity between the flexible membrane and the rigid sheet to be filled with transparent fluid.
Liquid lenses have many advantages, including a wide dynamic range, the ability to provide adaptive correction, robustness and low cost. However, in all cases, the advantages of liquid lenses must be balanced against its disadvantages, such as limitations in aperture size, possibility of leakage and inconsistency in performance. In particular, Silver has disclosed several improvements and embodiments directed towards effective containment of the fluid in the liquid lens to be used in ophthalmic applications, although not limited to them (e.g., U.S. Pat. No. 6,618,208 to Silver, and references therein). Power adjustment in liquid lenses has been effected by injecting additional fluid into a lens cavity, by electrowetting, by application of ultrasonic impulse and by utilizing swelling forces in a cross linked polymer upon introduction of a swelling agent such as water.
Commercialization of liquid lenses is expected to occur in the near future, provided that some of the limitations noted above can be remedied. Even so, the structure of prior art liquid lenses is bulky and not aesthetically suitable for consumers, who desire spectacles having thinner lenses and spectacles without bulky frames. For the lenses that operate by injection or pumping of liquid into the body of the lens, a complicated control system is usually needed, making such lenses bulky, expensive and sensitive to vibration.
In addition, to date, none of the prior art liquid lenses provides the consumer with the ability to introduce the liquid into or remove it from the lens chamber so as to himself change its volume in order to vary the power of the lens. In addition, none of the prior art liquid lenses provides a mechanism to allow the consumer to introduce the liquid into or remove it from the lens chamber so as to himself change its volume in order to vary the power of the lens.